From the prior art it is known to provide a butterfly valve in the boost air duct of, for example, a supercharged diesel engine for help in starting up the engine. Such a valve is shut at least till the engine has fired, and then the valve is opened, at the latest, by the time the engine has run up to a speed outside the idling range. The valve is then kept open till the engine is shut down again. More specially, diesel engines with pressure wave superchargers must have a butterfly valve at this position working in this manner, as otherwise, if transfer of gas and air between the supercharging air and exhaust gas sides of the supercharger occurs during the start-up stage at the time of the first strokes of the engine on starting, such transfer of exhaust gas would be likely to cause exhaust gas to enter the boost air duct of the engine and cause immediate stoppage of the engine.
German Published Application No. 2,631,257 discloses an actuator for a butterfly air valve in the boost air duct of a diesel engine, more specially a diesel engine with a pressure wave supercharger. The actuator has a shut-off valve in an actuator line running from an oil pressure line of the diesel engine to an actuator space for an actuator piston of the butterfly valve, the other side of this piston being acted upon by a force at all times. The shut-off valve is designed to be moved in the opening direction by a piston (which is acted upon on one side by air from the boost air duct for moving the piston in the said opening direction, and acted on on its other side by a spring) and may only be closed when there is no oil pressure. The shut-off valve is actuated by the actuator piston and may be closed in the rest position of the actuator piston (in which the butterfly air valve is closed), when there is no oil pressure present in the system.